Telephone repeater



April 9 1935- F. A. HUBBARD TELEPHONE REPEATER Filed Sept. 1, 1953 2 Sheets-Sheet l INVENTOR EHub/oaco BY 4 ATTORNEY April 9, 1935.A F. A.`HUBBARD 1,997,218

I' TELEPHONE REPEATER Filed sept. 1, 193s. Y 2 siens-sheet 2 lNVENTOR Fggwam ATTORNEY Patented Apr. 9, 1935v y i'rELErnoNE nEPEa'rER Francis Alley. Hubbard, Maplewood, NVJ., yas-v signor to American Telephone and Company, a corporation oi New York Telegraph` l V` application september 1, 1933, serial No. 687,854 fr k11 claims.. (o1,y 179-170) .f

This invention relates to telephone repeaters,Y and more particularly to two-way repeaters of the so-called directional type, intended for use on irregular two-Wire telephone lines.VVV f One object of thev invention is to iobtain an effective gain vfrom the repeater larger than would be permitted by the impedance'balance or freturn loss between the lines to' which the repeaterv is connected and their respective balancing net# Works. Vi 1 f vAsecond object ofthe invention is to simplify the attenuation pads used in this type of repeater, and improve the meansof switching them.

.A furtherobjectis to minimize the harmful effects of noise on the circuit upon the operation of the repeater. j Y A In attaining these objects, the applicant, in general,- moldiiies an ordinary type` of two-way telephone repeater by introducing an attenuation pad in one side or the other of the repeatenso that there isY always suiiicient loss inthe singing path through the repeater to prevent objectionable circulating currents, although one side of the repeater is giving a substantial effective gain.

operated relay, the direction of operation of which depends on the direction from which ytelephonie currents are reaching the repeater. Thus the pad is removed from the side of the repeater which is transmitting the telephonie currents, and an equal pad is inserted Vin the other side. Electricalfbiasing means are provided'which hold the relay in the direction last operated until the arrival oftelephonic currents of a predetermined strength fromrthe other direction. Sincenoise currents may be strong enough toV reverse the relay. `during brief pauses in speech, aslow-release relay, is provided, also 'voice-operated but without bias, to reduce Vthe sensitivity of the 40 main relay during speech to a point Where noise currents will be unable to operate. it. f

Thev principle of operation of the directional repeater has been described in two copending applications, Serial Numbers 687,880v and 687,874,

" which areY assigned to the same assignee as the present application.. It may lbe briefly stated as follows: n

It is well known Vthat With the ordinary type of telephone repeater used on two-wire lines, the

gain that can be obtained is kdetermined by the impedance similarity between the lines to which the repeater is connected andthe correspondingV Y artiiicial lines or balancing Vnetworks with which the repeater is equipped. The measure ofthe impedance similarity at any frequency between These vpads are under the 'control of a voicel. `quencies than'the sum of thereturnlossesjsatmay result from the loss of these sounds.' Iv preoneof the linesand its networky iscalled the"V n return loss atthat frequency. If at any fre-l quency. the sum of the repeatergains in the two directions-isgreater than Athe sum-i ofithe return losses of theltwo lines against their networks, 5 the repeaterwill sing, thatis, setup a. sustained oscillation, 'at that frequency. 'l f Ordinarilya single 'krepeater'is given the same, gain inboth directionspsince substantially equal transmission desiredzjforv both subscribers. l0

However, gain is required in only one direction at a time, and the gain :in theother side of the repeater may bereduced, provided the; conditions v are interchanged' every timethe direction of conversationchanges.. This can be done by means l5 of'voiceffrequency devices,arranged to. recognize the direction from which telephonie currents are reachingtherepeatenand to modify thegains f 'ofthe two sides of the repeater accordingly. r The gaininthel direction of transmission'mayv then 20 be given avalue-which would result in singing if both sides of the repeater had that value;` if the .sumV lof that gainand 'the reducedgain in the other side is substantially less at'all freisfaetory resultsv will be obtained. A repeater so arranged iscalled a directional repeater.

Itf-Would be Vpossible 'to Vdisable entirely Vthe amplifying element in the reverse direction, as is usually done in 'suchdevices 'as' echo Suppressors.V This has thedisadvantage, howeven-that as weak speech sounds, lparticularly initial consonants, maybe unable togoperate the. device, consid-` erable distortion and. reduction in intelligibility fer to-makethe reduction in gain, in the reverse direction, just suibient at all frequencies to provide a reasonable marginagainst singing, and againstthe distortionwhich results from a near approach tosinging. Speech sounds Whichare tooweak to operate the device will then be attenuated, but notentirely lost. f Y The device described herein is similar to one of the embodiments disclosed in application Serial Number v687,874: already referred to, with certain distinctive-differences which willV appear inthe followingdetailed description, and which make the device more satisfactory under certain conV y dtions. The descriptionis to be read with reference to the accompanying drawings, which in- *50 dicate diagrammatically-Jand in part schematically the circuits and apparatus involved.v vLike reference characters designatelike parts in the two iig'ures of the drawings," whichrepresent two dif` ferent .embodiments off the invention, each4 of 1 amplifier-detectors of any well-known form, connected across the conductors F and F', respectively, and responsive to telephonie currents flowing therein. The `outputs of these amplifier-detectors are connected diiierentiallyto the polar A ized relayl, in such a waythat thedirection of operation of this relay is determinedby' the yrelative magnitudes of the rectified currents flowing in the. windings M and M. Since the amplierdetectors E and'E' are substantially similar, the' direction of operation of relay I 'will vbe determined'by the relative jmagnitudes of the .tele` phonic" voltages-applied' to their .1 inputs.; The armature-'of relay SI serves to short-circuit. one or theY othersof theattenuation padsjI-I, H', thus providing .the full gain. of thelamplifler D or D', while reducing the gain'in'- the opposite direction by the: insertion of' the pad.. .The pads as shown 'are designed togive greater losses at'high and low frequencies than lin the middle of 'the voice range, since the returnlossof :loaded cable'cir-v cuits, withV which such repeaters are most cornmonly used, has such acharacteristicwith fref quency. The .transformers T, T' inFigure l are used ifl necessary to avoid possible noise-induc tion from the lines due to thepresence of ther impedance'I-I or H' in onewire only.

The system as r just described constitutes -a .directional repeater, substantially Vas disclosed in application Numberv 687,874,already referred to. An important difference lies in the arrange'- rnent which permits the use of a single ,pad in each side of the repeater, instead of two pads in each side.. It is `necessary to the 'proper functioning of the device that the input terminals of the amplifier-detectors E, E' shall be. connected to the circuit at a point intermediate in the loss of the pads H, H'. The reason forthis requirement lies in the factthat, asitherefwill. ordinarily be a considerable degree ofpunbalance between the lines and theirV respectiveinetworksg te1e` phonic currents from either line will affecttboth amplifier-detectors, by transmission Vthrough the hybrid'coils. For. example, consider the circuit shown in Fig; i of the drawings, but assume that the inputs of the amplifier-detectors, EE were connected Adirectly across .transformers T and T', respectively. Suppose telephonie currents arrive from .line A. A portion 4of these currents will enter amplier-detectorE; the remainder will be amplified by amplifier D, and will enter hybrid coil C'j. A portion of these Vampliied currents will pass through vcoil C' and enter transformer T. If vthefloss in this passage through coil C', which depends upon the impedance balance :or return loss between line'A'V andv networkDQ is less than theV amplification of amplifierD, the currents entering transformer T', may be greater than those originallyfent'eringT. Thiscondition might occur without causing the rep eater to sing. In this casethe rectified currenti'n wind-v ing;M' would be .greater than that inf winding M, and the relay would reverse, putting the ampli- Acation in the wrong direction. .A'ssoon as this happened, the Vcurrents ii'owing through amplifier -reverse the deviceat all.

D would be reduced by the pad H, and the portion entering transformer T', and thus affecting amplierfdetector y E', would be smaller than those affecting E, and relay I would reverse once more; Relay I would, therefore, swing rapidly back and forth as long as speech continued, an obviously unsatisfactory condition.

Suppose, however, that the amplier-detectors E. and'E were connected directly Vacross the amplifiers D and D', respectively. The condition of Ahunting described in the last paragraph could not thenexist; but it might be impossible to Suppose the amplifierdetectors so connected in the circuit of Fig. 1, and the armature of `relay I against its .lefthand contact, as indicated. Speech currents from line A will then receive amplification, and will '.leave the relay in that position, since the cur- .rents reaching amplier-detector E will be attenuated by pad H', and will be less than those Ventering E. In the case of currents arriving over line A',uhowever, if` the loss through coil C is less than thegain through amplifier D', the input to amplifier-detector Elwill be greater than that to E', and relay I will be held in the wrong position. To avoid these conditions,'it is desirable to be able to connect the amplifier-detectors at suitable Vintermediatepoints within the pads. To do this Yseveral intermediate points on the retardation coils, 4as indicated, so that theposition of the amplifier-detectors can be adjusted to suit the particular conditions encountered. For example, if it were known that the balanceV against Vone of the lines would, in general, be substantially better than'against the other, it might be of advantage to use different` settings` on the two retardation coils.

, yInFig. 2 of the drawings is shown a different method of connecting the pad into the circuit, and of obtaining the desired intermediate point for the ampliiier-detectors. The pad is connected inductively, through the transformer U, insteadv of directly; and toavoid the necessity of` usingthe insulating transformer T, the line windings are divided equally vbetween the two sides ofA the circuit, as indicated, thus providing a longitudinally balancedcircuit. Toobtain the 'desired points of connection for the am'- plier-detectors, it would be possible to bring out taps on one or both of the line windings of the coils U and U. As this would result in' some- Whatcomplica'ted and expensive coils, however, the method indicated in Fig. 2 is preferred. In this, each of the coils U,'U is of the familiar hybrid coil construction, similar in general, to coils C and C,`with they center point of each line winding accessible. The inputof amplifier-detector E is Yconnected'to these center points on coil U, and similarly with Eto'coil U'. The inputirnpedance of each amplifier-detector must be widely different fromthe impedance of the Loomis as seen through the hybrid coil C. If'these two v impedances are equal, substantially the same voltage will be applied to the input ,ofA the amplier-detector E whether' `the secondary of transformer U is short-circuited or not. Under these, conditions all the loss of pad H is eectively beyond the point 10i connection of `arriplier-detector E, which as already rexplained may result in instability of relay I'and-unsatisfactory operation. The `same is ofcourse true of the other side ofthe repeater. If, however, the impedance looking towards coil C is substantially less than the input impedanceof amplifier D, the voltage applied tothe'amplier-detector E will be less when the secondary ofcoil Uis closed through pad" Hthan when it is vshortcircuited, which means that under these conditions the amplifier-detector is connected, as de# siredatan intermediate point in the losso'f pad H, the exact position depending onthe relation between the impedances. Therefore, a Vari" able resistance V is provided bridged-across the circuit, as indicated, for the purpose of making the impedance on that side of coil U less than the input impedance of the amplifier D. By adjusting the Value of resistance V, the point of connection of amplifier-detector E can be effectively brought to any desired intermediate point in the loss of pad H, as was done in the circuit of Fig. 1 of the drawings by means of the taps on retardation coil J. Resistance V will of course introduce a loss in the circuit, but it is a xed loss which can be compensated by an increase in the gain of amplifier D.

The same discussion applies to the other side of the repeater, in-which resistance V' is provided for the samepurpose as resistance V. It is desirable that the loss introduced by resistances V and V be substantially the same, so that the gains of the amplifiers D and D may be substantially equal.

Another feature of the invention is the rpro-- vision of adjustable electricall bias to hold the armature of relay I in the direction last operated, whether or not telephone currents are arriving from that direction, until thearrival of such'currents of a predetermined strength from the'other direction. The biasing flux must of course be reversed in direction when the armature of the relay changes from one contact to the other. This could be accomplished directly through the contacts; but in this case the biasing current must be made and broken by the contacts, and the resulting clicks would make it impossible to connect the contacts directly to thepads. A n' alternative method is to rely onthe pull of the' permanent magnet of the lrelay to Vhold the armature against preaches. This leaves the relay contacts free of direct current, so that they lmay beconnected Vto the pads, and it has the advantage of sim plicity. It lacks however, the reliability and ready adjustability of an electrical bias.` ItA is preferred to use the electrical bias, whichit is proposed to obtain by auxiliary Lmeans without causing clicks by making or breakingany ,appreciable whichever 'contact it Y ap-V current through the relay-contacts. Fig. y1 and Fig. 2 of the drawingk show somewhat different means of accomplishing this, each ofV which will have advantages under certain conditions.

InFim 1 the bias is controlled by an auxiliary relay; K, :similar ,to relay I and adjustedto be as closely like'it -aspossible in operating charactere istios. `The operatingwindings N and N are connected in serieswith the operating windings M and- M', respectively, of relay4 I, and the biasing windings Q and Q' in parallelwith the biasing windings P and Pf, respectively, of relay I. When relay I is operated Vto the left, by telephonie currents arriving from Vline A, relay K is simultanef ously operated-t0 the left, and biasing currents A ow through the left-hand contact of relay K and through windings P and Q in'parallel to ground.y The uxes due-tothese biasing currents are such as to hold both relays firmly against their left-hand contacts until the arrival of telephonie currents of a ypredetermined intensity from line A-,.when both relays` operate to the right, breaking the' current through biasingfwindings P 'and .Q, and rsending current through the opposite biasing windings Pand Q which arek 'i so poledl as to hold the relays against their respective right-hand contacts. The intensity of the telephonie currents required to reverse the relays, that is, the sensitivity of the device, is largely determined by the strength of the biasing iiuxes:

the biasing currents of the two relays can be inden A pendently adjusted by the resistances Z and Z.

S and S' represent lamps, yto give the maintenanceforces a. visual indication'of the direction Y in which the device is operated. If the relays are the drawings are somewhat different. The auxiliaryf relay is omitted,rand the current for the biasing windings P and P', is supplied from battery Y, through the 'adjustable resistance Aa, lampsS and S', and the plate circuits ofk vacuum tubesV W and W respectively. The grids of tubes` W and W arek connected to the right-hand and left-hand contacts, respectively, of `relay I. Bat- If one, ofr

tery X supplies a negativepotential to the arr'na-` ture of relay I, .of sumcient'magnitude when ap:

plied to the gridof either of the AVacuum tubes W or VW'to block the flow of plate current in that tube. However, on account of the very high impedance of the grid circuitl of a vacuum tube, i

will carry no current. Current will iiow, however,

through'winding" P, lamp S, andthe plate circuit of vacuum tube W to ground, Relay I will thus be held against its left-hand contact, and lamp S will be lighted; Similarly, when telephonie currents of sufcient strength arrive from line A to operate relay I` to the right, current will flow through winding P' of relay I; lamp S', and the plate circuit of vacuum tube W' to ground, while Athe current through winding P will be cut off by the application of the potential of battery X to the grid of tube W. Thus the relay armatureV willbe heldagainst its right-'hand contact, lamp S will be extinguished andlamp S'Vlighted. The

' vide, in the cases where noise may be serious, aV

acteristic of noise currents in a telephonesystern is that they are fairly steady inintensity, as compared .withthegwide fluctuations vof 'voicey currents. It will seldom happen that noisecurrents will be comparable in intensity'withathe peaks of Voice currents, which' correspond1 to vowel sounds; but they may occasionally be greater than intermediate portions "off speech. currents corresponding" to consonants'. .Since the .directional repeater as described hereinis at all times sensitive to telephonie currents from bothY direcl tions, it may happen that steadyY noise fromfthe direction `opposed tothat `from which 4speech is reaching the repeatermay bef able to reverse the relay during pauses in the speech, or whenthe speech currents are of 10W intensity. While the next succeeding vowel sound Vwould usually'be` able to operate the device in the rightYdirectio-n, the reversal, though brief, would introduce a certain amount of undesirableclipping 4and distortion'of theconsonant sounds. 1 v. rvi

To minimize this eifect, itis vproposed to pro# device which will -reduce .the sensitivity of ythe main relay while speech currents, of a certain intensity are owing, and Willmaintain that re- -duced sensitivity for a briefvperiod .sufficient to bridge the usual time between vowel sounds in speech. This reduction'in sensitivity will be great enough to make it unlikely that no-isecur'- rents cancause operation, so that during normal speech there will seldomv be a' reversal dueto noise, except incase of apause longenough for the sensitivity-reducing device to release'. There will, of course, be some sacrice inv speed of reversal when speech from theother endof the circuit begins immediately, 'especially vif the initial speech currents are of low'intensity, Vdue to consonant sounds' This will result in some 'slight increase in clipping of initial consonant sounds, with consequent distortion. Under severe noise conditions, however, thismay be considerably less serious than the effect of frequent noise reversals. rIhere will be no chance of false operation -due to the 4reduction in sensitivity, sincethis vreduo tion is the same for bo'th directions oftra'nsmis'e sion. Two slightly different embodiments oflthis arrangement are shown in the two-:figures ofY the drawings, each having certain advantages underY particular conditions. In Fig. 1,jL designates a slow-release relay, the sensitivity of which is considerably less than thatoffthe polarized relays around the operatingwindings M, NandM, Nl

of relaysv I and K, therebyreducing the-sensitivity of vthose relays 'by such anamountthat they are not likely to beoperated by noise currents.. lDue to the slow-release feature ofA relayk `L, obtained by any'well-linown, means such as a copper band around the pole piece, the. low,r

sensitivity condition of relays vI .and K will be maintained for a brief period after" the currents in the windings of relay L have fallen `belowthe value necessary to hold it operated."I'his`-will fers somewhat.

serve to protect the repeater vfrom false reversals due to noise during brief pauses in speech, such as spaces between vwords or syllables.

f Assume that speech currents are arriving from line A. If these currents are of suicient intensity-to operate relay L, they will rst operate relays I and K, since relay L is much less sensitive. As long as normal speech continues from line A, relay L will be heldoperated by the rapidly recurring peaks'of current representing vowels, Now assume that the speech' from A ceases, and immediately speech from A begins, before relay L releases. The adjustment of relay L with respectto relays I and K is such that if the speech currents arey too weak to reverse relays'I and K in theircondition of reduced sensitivity, they will alsojbe...too weak to hold relay L operated. If the initial speech'currents from A' are weak, therefore; relay L will release after a brief interval, and*restorerelays I and K tofull sensitivity; ifthe speech currents are strong enough to hold relay L" operated, they will reverse relays I and K'evenat 'the low'sensitivity. Thus the only interference with proper operation on speech will be on initialcurrents of low intensity, for the-brief' period of holding for which relay L-is adjusted. Y

"Theiarrangement in Fig. 2 of the drawings is very similar, though the point of application dif- In this figure, Bb designates a slow-release relay, whose sensitivity is adjusted with respect to that of relay I along the lines just discussed; that is, the currentnecessary to hold it operated is less than that required to operate relay I at its minimum'sensitivity. The windings of relay Bb are connected in series with Vthe operating windings of relay I. When relay Bb operates, its armature short-circuits a certain part of the resistance Aa., thereby increasing by a predetermined amount the biasing current through winding P or P of relay I, and thus decreasingthe sensitivity Aof that relay. The amount of this change in sensitivity, as well as the maximum sensitivity, may be adjusted by means of resistance Aa. The` operation of relay Bb in Fig. 2 ofthe drawings is exactly analogous to that already explained with regardto relay L in Fig. v1. I f While the invention has been disclosed, for the purposes of illustration, in certain specic embodiments, it will vbe understood, that it may be embodied in other and diierent forms without departing from the spirit of the invention which is defined inthe appended claims.

What is claimed is:

1. InatWo-way 'telephone repeater including two one-way paths adapted for transmission in opposite directions;'a voice-operated polarized relayfadapted to transfer attenuation from one path of the repeater'to the other, electrical means for biasing'the said polarized relay in the direction last operated without passing current through the contacts ofthe said relay, and means for so adjusting the said bias that a predeterminedvalue of telephonie current will be required to operate the said polarized relay in the reverse Y direction.

2. In `atwo-way telephone repeater including two 'one-way paths adapted' for transmission in relay adapted to transfer attenuation from oneA path of the repeater to the other, a thermionic device controlled by the position of the armature of the said relay and adapted to supply bias to the relay to hold it in the direction last operated, and means for so adjusting the magnitude of the said bias that a predetermined value of telephonie current will be required to operate the said relay in the reverse direction. Y

4. In a two-way telephone repeater, two separate unidirectional paths for transmission in the two directions, an amplifying element and an attenuation element in each path, a voice-operated device associated with each path, switching means differentially connected to the-two voiceoperated devices and adapted to remove one or the other of the said attenuation elements from its associated path, and adjustable means for connecting each voice-operated device to its associatedpath at any desired intermediate point within the corresponding attenuation element.

5. In a two-way telephone repeater, two separate unidirectional paths for transmission in the two directions, an amplifying element in each path, an attenuation element inserted in series in each path, a voice-operated device connected across eachpath, adjustable means for so locating the point of connection of each Voice-operated device to its associated path that a predetermined portion of the loss of the corresponding attenuation element is effectively on each side of the said point of connection, and means differentially controlled by the two voice-operated devices to shortcircuit one or the other of the two attenuation elements in accordance with the direction of arrival of telephonie currents at the repeater.

6. In a two-way telephone repeater, two separate unidirectional paths for transmission in the two directions, an amplifying element in each path, an attenuation elementinserted in series in each path, an impedance element in parallel with each attenuation element, a voice-operated device associated with each path, means for connecting each voice-operated device from any of several points on the associated impedance element to the opposite side of the associated path,

'and means differentially controlled by the twor voice-operated devices to short-circuit one or the other of the two attenuation elements in accordance with the direction of arrival of telephonie currents at the repeater.

7. In a directional repeater for use on irregulary two-wire telephone lines, including two one-way paths adapted for transmission in opposite directions and a voice-operated switching system adapted to transfer attenuation from one path of the repeater tothe other, the method of protecting the repeater against false operation due to noise, which consists inreducing the sensitivity of the switching system when the received telephonie currents exceed a certain maximumintensity, and maintaining the reduced sensitivity for a predetermined period after the currents fall below that intensity.

8. In a directional repeater for use on irregular two-way telephone lines, including twoone-way paths adapted for transmission in opposite`r directions and a voice-operated switching system adapted to transfer attenuation from one path of .n

the repeater to the other, the method of protecting the repeater against false operation due toV noise, which consists in reducing the sensitivityV of the switching system when the received telephonic currents exceed a certain maximum intensity, and maintaining the reduced sensitivity after the currents fall belowrthat intensity for a period corresponding approximately'tothe interval between words in average speech. y f

9. In a two-way repeater including two one-way paths adapted for transmission in opposite'directions, a voice-operated switching device adapted to transfer attenuation from Lone path of the repeater to the other, means for reducing the sensitivity of the said device'when the received telephonic currents exceed a certain maximum intensity, and means for restoringthe original sensisubstantially lower sensitivity than the said device and adapted to reduce the sensitivity of that device, and means to restore the original sensitivity of the said device a predetermined time after the telephonie currents have decreased below the value necessary for the operation of the said relay.

1l. In a two-way telephone repeater, two separate unidirectional paths for transmission in the two directions, an amplifying element and 'an attenuation element in each path, a voice-operated device associated with each path and responsive to telephonie currents flowing therein, adjustable means for connecting each voice-operated device to its associated path at any desired intermediate point of the corresponding attenuation element, a polarized relay differentially connected to the two voice-operated devices and adapted to remove one orthe other of the attenuation elements from theV associated path, adjustable electrical means requiring no current` through the contacts of the said relay forv holding the ,relay in the direction last operated until the greater than a predetermined minimumintensity, voice-operated means for reducing the sensitivity 'arrival of the. repeater of telephonie currents of the said relay when the'telephonic currents are greater than a certainmaximum intensity,

and means for restoring the original sensitivity.

ofv the said relay a predetermined time after the telephonie currents have become less than the said maximum intensity.

` FRANCIS A.l HUBBARD.

25 Vtivity of the device at a predetermined interval*` 7 after the telephonie currents fall below that in- 

